Hydrocarbon fractionating



Patented May 10, -1949 HYDRooARBoN FRAo'rIoNA'riNG George W. Gross, Linden, N. J., assignor to Standard Oil Development' Company, a corporation of Delaware Application December 27, 1946, Serial No. 718,851

1 Claim.

This invention relates to the fractionating of hydrocarbon mixtures and in particular to fractionation of normally gaseous hydrocarbons from oil-field and petroleum refinery mixtures.

In petroleum refining today, large volumes of hydrocarbon gaseous mixtures are obtained. From these mixtures many valuable gaseous compounds are recovered. Local conditions usually determine the most advantageous means of processing these mixtures to separate desired constituents. The present invention is concerned with the processing of normally gaseous hydrocarbon mixtures for the separation of individual constituents by fractionation under pressure. The particular feature of the invention is the employment of side stream products to provide refrigeration for the liquefaction of the highly volatile overhead gaseous products. This use of constituents of the feed supply to obtain the refrigeration for convenience of processing more Volatile constituents overcomes the prior art need of external refrigeration.

In order that the invention may be more fully understood, the following specific description in connection with the accompanying drawing is presented as an embodiment, but not a limitation, of the invention.

For purposes of illustration, treatment of a feed stock obtained from an oil-field plant will be taken. A typical composition of such a feed stock is as follows:

In this illustration, a feed stock of the above composition is introduced through line 4| to a compressor 42 wherein suicient pressure is applied to condense the easily liqueflable C5 to C'z hydrocarbons. The stream of uncondensed gases and liquefied hydrocarbons are removed from compressor 42 through line 44 and are introduced to separation zone 43 wherein the liqueed hydrocarbons are separated from the uncondensed gases. Thus the liquefied hydrocarbons constituting the C5 to Cv and higher hydrocarbons are removed from separation zone 43 through line 45 while the remaining uncondensed gases are passed through line II to the compression equipment 23. In equipment 23, the gas is compressed to a pressure of about 440 pounds per square inch. The substantially liquefied composition is then (Cl. 18S-114.6)

2 cooled in heat exchanger 32 and passed into about the middle of fractionation tower |0. This tower is usually of the bubble plate type. The fractionation tower in this particular illustration contains bubble cap plates of about 30 plates in number. The tower I0 is shown as being equipped with an overhead vapor line I2, an upper liquid side stream exit line I3 connected to a cooler I4 and return line l5, reboiler inlet line |65, reboiler I'I, reboiler vapor return line I8, a refiux line I9 and side stream exit line 20, and a condenser 2 I. Heat is shown as being maintained in the column I0 by means of the reboiler system I6, I'I and I8. Temperature control of about F. in the upper part of the tower is maintained by the cooler system I3, I4 and I5 and the amount of reflux through line I9. The stream admitted ythrough line l5 has a temperature of 90 F. and the reflux stream through line I9 has a temperature of 50 F'. A pressure of about 430 pounds per square inch is maintained upon the tower and the directly connected equipment, the condenser 2|, cooler I4 and the reboiler I1. n

The overhead product passing from the tower at F. through line I2 enters condenser 2|. The condenser 2|' is cooled to about 35 F. by the vaporization of the side stream material removed from the tower from a plate located near the exit of line 20. To effect this vaporization, the pressure upon the side stream product may be reduced as shown in the device 30, or as in this embodiment, partly in device 3| and partly in condenser 2|, to about 20 pounds per square inch. The composition of this stream is:

. Mol Percent C2 3.0 C3 85.2 1-C4 n C 11.8

Mol Percent C1 11.2 C2 20.6 C3 63.5 -C4 1.6 lil-C4 3.1

The vapor product from the drum 24 is passed through line 25 for further processing. The com- `position of this stream in this particular embodiment is:

Mol Percent C2 20.0 C3 21.1 -C4 0.2 Il--C4 0.3

The distillation residue is removed through line I6 into the reboiler Il. From the reboiler l1 portion of the liquid material is returned through line I8 to provide the necessary heat for the operation of tower I0. The temperature of the stream admitted through line I8 is about 330 F. The remainder of the liquid is passed through line 26 for further processi-ng. The composition of this stream is:

Mol Percent n-C4 27.3 -Cs 13.3 n-C5 8.5 C6 10.1 C7 l13.8

The applicants invention is thus concerned with the processing in a low temperature gas fractionating tower requiring overhead refrigeration. The applicants invention is in regard to the use of a side stream material for supplying the refrigeration largely through the reflux stream instead of the conventional method involving external circuits for one or more of the streams supplied to the tower. Furthermore, temperature control can be easily effected in the distillation zone, according to the invention, by interadjustment of the volume of the reflux stream and the amount of pressure reduction on the side stream fraction. Also, the -vaporized side stream may be used to cool the feed supplyafter compression as shown in the drawing by the circuit indicated by reference numerals 3|., 32 and 33.

Processing according to the applicants invention provides refrigeration together with the fractionation of the lconstituents of the hydrocarbons mixture. The integral refrigeration-fractionation system thus consists Aof withdrawing a liquid side stream from the tower, using the side stream as a refrigerant and recycling the refrigerant vapors after passing through compression equipment to the fractionation tower. The refrigerant vapors may be added to the-feedgas at any stage, depending upon the refrigerant pressure and temperature required at the reflux condenser. To minimize compression requirements, the refrigerant vapors should be added to the feed gases at the highest pressure level. Heat removal for condensing the ma-jor portion of the reflux is provided by the side stream cooler at a temperature level not requiring refrigeration.

The integral refrigeration-fractionation system of the applicants -invention has many processing advantages. Of these, the main advantages are:

1. A substantial investment saving is obtained in comparison with the conventional system of using external refrigeration system.

2. Greater simplicity in the tower operation and space requirements is realized because of the i compactness of the fractionating tower and integral refrigeration system, and the relatively few equipment pieces.

.3. In the conventional closed-type refrigeration system, trouble is frequently encountered due to the accumulation of light wild gases. The integral refrigeration system is relatively free of this dificulty.

What is claimed is:

A process for separating Ia complex mixture of C1 to yCa hydrocarbons which .comprises .subjecting said mixture to apressure suficient to liquefy Cs to Cs hydrocarbons, removing the liquefied C5 to Cs hydrocarbons leaving a residual mixture of C1 to C4 hydrocarbons, subjecting the said mixture of C1 to C4 hydrocarbons to a pressure sufficient to substantially liquefy the mixture, passing said liquid mixture into a fraction-ating zone, operated at about 430 p. s. L, separating from said fractionating zone an overhead gaseous fraction comprising principally C1 hydrocarbons at a temperature of about 105 F., two liquid side stream fractions comprising principally C3 hydrocarbons, and a bottoms fraction comprising principally C4 hydrocarbons, reducing Vthe pressure on one of the said liquid side stream 'fractions to about 20 p. s. i., to eiect substantial vaporization, cooling the overhead gaseous fraction by indirect 4heat exchange with a part of -said vaporized side stream fraction,

A cooling the incoming feed stock to the fractionat ing zone with another part of the said -vaporized side stream fraction, condensing a portion of the cooled overhead gases providing a gaseous product and a condensed liquidr.emoving from said cooled overhead fraction the gaseous product, recycling the condensed liquid as reflux into the fractionating Zone at a temperature of about 50 F., compressing the vaporized side stream vmaterial after said heat exchange and recycling said compressed side stream product into said fractionating zone and cooling the second liquid side stream fraction to a temperature of about 90 F. and passing the cooled second liquid side stream to an upper part of the fractionating zone above the pointat which the second liquid side stream was withdrawn.

GEORGE w. GROSS.

REFERENCES CITED rhe following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,074,094 .Blau Sept. .23, 1918 1,415,058 Schill May 9, 1922 1,924,196 Miller Aug. 29, 1933 2,000,992 Schlitt May 14, 1935 

